In recent years, as an optical element that enables high-speed (100 Gbps or the like) and high-capacity communication, dual polarization-quadrature phase shift keying (DP-QPSK) described in Patent Literature No. 1 has been put into practical use. The above-described optical element includes a substrate on which two QPSK modulation units (having a waveguide structure called a nested structure) are formed in parallel, a light focusing element, a polarization combine element (Patent Literature Nos. 2 and 3), and the like. In addition, the above-described optical components are disposed in a housing section formed of a metal such as stainless steel. The housing section is made up of a box portion and a lid portion that covers the box portion and is air-tightly sealed.